Undercarpet wiring systems for power, data and telephone applications have been in commercial use for several years. One such wiring system is manufactured by the assignee of the subject invention and is sold under the trademark VERSA-TRAK. The wiring system for power applications, for example, typically includes a flat cable comprising three, four or five conductors encased in a jacket or casing of insulation. A conductive shield overlies the cable for mechanical and electrical protection and is electrically connected to the ground conductor such that the shield and ground conductor are at the same ground potential. In the VERSA-TRAK.RTM. system, the ground shield is connected intermittently along the cable length to the ground conductor by welds. An insulative film is often attached to the bottom of the casing for further cushioning and abrasion shielding. U.S. Pat. Nos. 4,219,928 and 4,283,593, both of which are assigned to the assignee of the present invention, describe a known electrical cable asssembly for use in undercarpet wiring systems.
In use, the cable assembly is adapted to be placed on a floor surface in commercial buildings and thereafter covered with carpet squares. As the cable with shielding is relatively thin (generally less than 1/16 inch), the wiring system remains unobtrusive once installed. It has been found through experience and tests that certain elements in the environment to which the undercarpet wiring system is exposed could present problems in use. These elements include dirt particles or other debris and water. During installation or use, particles of dirt may get lodged between the cable casing and the metallic shield or between the cable and the floor thereby increasing the potential for puncturing the cable and either damaging the system or shutting down the system by tripping a circuit breaker through a short circuit. In addition, and in particular where the undercarpet cable is placed on a concrete floor on the grade level of a building, water may be vaporized from the concrete surface and be retained under the carpet after condensing. The water may combine with the chemicals in the concrete to form an alkaline solution consisting mainly of calcium hydroxide. Calcium hydroxide and other alkaline solutions are known to attack certain materials, such as polyesters, which are commonly used in the undercarpet cable insulation because they have good dieletric and hardness properties. It is known, for example as disclosed in the above-referenced patents, to form a composite jacket of insulation with a harder layer of polyester, such as MYLAR film (MYLAR is a trademark of E. I. Du Pont de Nemours) on the jacket exterior with a softer layer of polyvinylchloride on the interior of the jacket. It is also known that undercarpet cables use one or two layers of MYLAR film as the insulation jacket, the film being laminated to the conductors.
Steps have been taken in the industry to ensure safe and reliable undercarpet systems. For example, as a means of providing a moisture barrier, especially on grade level concrete floors, a separate strip of polyvinylchloride (PVC) is laid on the floor beneath the cable (or beneath the abrasion shield). This PVC strip is several inches wider than the width of the cable and serves as a moisture barrier to water or calcium hydroxide when present on the floor. This PVC strip further serves as an additional cushion for minimizing puncturing of the cable from dirt particles between the cable and the floor.
Other attempts not presently adapted commercially include efforts at sealing the edges of the cable assembly so that water, dirt and other debris may be kept outside the cable assembly. One such approach is shown in U.S. Pat. Nos. 4,417,096 and 4,319,075, both issued to Willette and assigned to AMP, Incorporated. In addition to costs, obtaining entrance to the sealed cable assembly for making taps or splices, and then re-sealing the cable assembly present problems.
While current undercarpet systems have been improved for safety and reliability, there is a continued need for providing solutions to the water and debris concerns in a cost effective manner.